Method of producing low carbon high chromium cast iron



Patented Dec. 7, 1937 UNITED STATES.

PATENT OFFICE METHOD OF PRODUCING LOW CARBON HIGH CHBIOMIUM CAST IRON NoDrawing.

Application March 14, 1935,

Serial No. 11,074 v 6 Claims. (01. 75-427) This invention relates to themanufacture of iron castings, and more particularly to a method ofproducing, by cupola melting, high chromium cast irons of relatively lowcarbon content, and

to an addition alloy used in said method.

The advantageous combination of physical and chemical properties of highchromium cast irons has led in recent years to their widespread useand'it has been found that high carbon content 10 has a deleteriouseffect upon the desired physical and chemical properties of such castirons. Specifically, high carbon content greatly lowers the resistanceto mechanical or thermal shock, decreases machinability, and lowers thephysical strength. Because of the Strong afinity of chromium for carbon,such cast irons, if produced by melting in the ordinary foundry cupolawith carbonaceous fuel, absorb large amounts of carbon and are for manypurposes quite useless. For these reasons, it has been necessary toproducehigh chromium cast irons of relatively low carbon content bymethods wherein no substantial amount of solid carbon is in contact withthe molten metal-such as melting in electric furnaces, oil firedfurnaces, or the like.

An object of the present invention is to provide means whereby highchromium cast iron of relatively low carbon content and possessing thedesired physical and chemical properties can be produced by melting theraw materials in an ordinary coke fired foundry cupola.

Another object of the invention is to improve the recovery of metallicchromium in the cupola melting process,-which has the further advantagethat the melting rate can be maintained or increased, even when meltinghigh chromium alloys, without entailing prohibitively large losses ofchromium.

High chromium cast iron containing, for example, about 15% chromium andabout 2.5% silicon, produced by ordinary foundry cuploa melting methods,has a carbon content of upwardsof 3.50%, and usually well over 3.75%.The 45 desirable upper limit of carbon content of such an alloy is about3.00%. Hence, a method of decreasing the carbon content effectivelywithout adversely affecting the physical and chemical,

properties of the product would be of great value.

It is known that the use of a large proportion of steel in the chargeand the use of high blast pressure tend to lower the carbon content ofcupola cast mm, but these expedients do not suffice to lower the carboncontent of chromium u irons to the desired maximum percentage.

Because of its effect in lowering the solubility of carbon in ironalloys, it might be expected that an increase in silicon content of theproduct would solve the problem; but in: practice the amount of siliconrequired to inhibit pick-up of carbon, to any useful degree, is so greatthat the entire character of the cast iron product is altered.

This invention is based on my discovery that if silicon and copper aremelted together in close association with chromium, the inhibitingeffect on carbon pick-up by the chromium is so pronounced that castingsof required chromium, carbon, and silicon contents can be produced,while the physical and chemical properties of the castings are improvedthrough the alloying efiects of the silicon and copper.

The invention comprises adding chromium in the form of a complex alloyor metallic mixture containing in close association the requiredproportions of chromium, silicon, and a relatively noblegraphite-inducing metal such as copper or nickel which inhibitsoxidation under the conditions'obtained in the cupola. This additionalloy or mixture is introduced with the metallic charge as it is fed tothe cupola, and melts with the charge in the melting zone of the cupola,alloying with the molten iron as it is formed, It is desirable to use alow carbon iron charge and high blast pressure,'but these expedients arenot usually essential to the obtaining of a product suiliciently low incarbon if the alloy is added in Per cent Cr Si Remainder substantiallyiron Composition of castings t Pu'oont Pu'cont Pcosnt Pcoont Cr Cu s1 MnPhysical properties Transverse strength t m aw. Hm NO. n

.in. Strength, Dellocem Dodcolbafer tion, lbagr tion,

sq. 11. inches sq. inches The decided lowering of carbon content, andthe great increase in desirable physical properties brought about by themethod of the invention, are obvious. If in this instance the efiectupon carbon solubflity of silicon alloyed with cast iron had alone beendepended upon to lower the carbon content, it would have been necessaryto raise the silicon from about 2.5% to about 6.5% which would havecompletely altered the physical properties of the castings and made thempractically useless. These low carbon castings, actually made in thecupola, have in general the properties and composition, includingsilicon content, of similar castings made by electric furnace methods,but they differ very greatly from such castings made by usual cupolamelting methods.

The following data from tests with a small cupola further illustrate theadvantages of the invention:

cated by the above data is an important advantage of the method of theinvention. Theloss of chromium in such operations, if the chromium isadded as free lump ferrochromium, is ordinarily on the order of thirtypercent, and with high blast pressure it may increase to as much asseventy percent, as indicated by the foregoing data. I have found thatwith the complex alloy of the invention, the presence of copper andsilicon together is efiective in decreasing the oxidation loss of bothsilicon and chromium, to such an extent that for given blast pressurethe loss is reduced approximately by half. This oxidation-resistantcharacteristic of the addition alloy of the invention makes it possibleto operate the cupola at high blast pressure without excessive loss ofchromium, which is an important factor in obtaining lowest possibletotal carbon content, as well as in economic operation of the cupola.

That copper exerts estimate and distinct influence upon the inhibitionof carbon pick-up was demonstrated by two melts, otherwise similar, in

the nrst of which chromium silicon alloy without copper was used, whilein the second melt the preferred proportion of copper was added. Thefirst melt showed 2.20%and the second 1.8% carbon pick-up.

In the development of the method of the invention I have'i'ound that theaddition alloy may take the form of a true alloy of the constituentmetals, or mechanical mixtures of them or of alloys containing them. Inthe case of mechanical mixtures the metals may be bonded in any suitablemanner or placed in metal containers to assure melting in proximity toone another.

The preferred range of composition of the addition alloy or metallicmixture is within approximate limits:

Cr=40 to 75% Si=5 to 15% Cu=5 to 15% C=less than 2% Fe=25 to 50%(remainder) As previously indicated, a true alloy of the metals issomewhat preferable but by no means essential, a suitable additioncomprising a briquetted mixture of ferrochromium, ferrosiliconandcopper, or copper silicon, chromium silicon, or the like. Low carboncontent is preferred in all cases.

While the invention has been described in terms of the use of copper asan alloying agent it is to be understood that within the scope of theinvention copper and nickel are substantial equivalents.

I claim:

1. As an article of manufacture for adding chromium toa cupola meltingcharge, a briquetted metallic mixture comprising a binder and metallicingredients having the approximate composition range: 40% to 75%chromium; 5% to 15% silicon; 5% to 15% copper; carbon in an amount lessthan 2%; remainder iron.

2. As an addition alloy for adding chromium to a cupola melting charge,a briquetted metallic mixture comprising a binder and a metallic alloyof the approximate composition range: 40% to 75% chromium; 5% to 15%silicon; 5% to 15% copper; carbon in an amount less than 2%; re-

mainder iron.

3. As an article 01 manufacture for adding chromium to a cupola meltingcharge, a briquetted metallic mixture comprising a binder and metallicingredients having the approximate composition range: 40% to 75%chromium, 5% to 15% silicon; 5% to 15% nickel; carbon in an amount lessthan 2%; remainder iron.

4. As an addition alloy for adding chromium to a cupola melting charge,a briquetted metallic mixture comprising a binder and a metallic alloyof the approximate composition range: 40% to 75% chromium; 5% to 15%silicon; 5% to 15% nickel; carbon in an amount less than 2%; remainderiron.

5. Method of producing,'by direct melting in a carbonaceous fuel-firedcupola, high-chromium cast iron having an abnormally low carbon content;I which method comprises adding to the charge to be melted, at least amajor fraction 01' the chromium in the form of briquets containingmetallic chromium, silicon, and metal selected fromthe group consistingof copper and nickel; the metallic ingredients of said briquets being soproportioned that the percentage of metallic 75 2,101,426 chromium isgreater than the sum of the perof closely associated metals, whichmixture has the approximate composition: 40% to 75% ohm mium; 5% to 15%silicon; 5% to 15% metal selected from the group consisting of copperand nickel; carbon in an amount less thanv2%; re- 5 mainder iron.

8 O. BURGESS.

